In recent years, light emitting diodes are widely adopted in many applications. The improvement of the luminous intensity for light emitting diodes has been greatly researched. There are many new developments in the structure of light emitting diodes that provide better quality and higher light intensity. Among the newest light emitting diodes, many of them are based on a double heterostructure comprising an upper cladding layer, a lower cladding layer, and an active layer between them. In general, the light emitting efficiency is quite limited because of the high resistivity in the upper cladding layer that makes the spread of the electric current difficult.
Several techniques and structures have been developed to improve the luminous efficiency for the light emitting diodes. Most of the approaches that have been developed focus on forming high conductivity window layer or layers on top of the double heterostructure for the purpose of increasing current spreading. U.S. Pat. No. 5,008,718 presents a high energy band gap and transparent window layer to improve the luminous intensity of a light emitting diode. U.S. Pat. No. 5,359,209 describes a double layer window structure including a low energy band gap layer and a high energy band gap layer.
Although the luminous efficiency of a light emitting diode has shown significant improvement in recent years, higher light intensity is still strongly demanded in many applications. In addition, better and more reliable structure of light emitting diode with less structural defects and longer life time is also very important.